The present invention relates to window glass antennas for motor vehicles and, more particularly to an antenna arrangement in which a defogging heater conductor formed on a window glass of a motor vehicle serves as an antenna for receiving radio signals.
An antenna arrangement of this kind is known in the art. In order to assure that a sufficient amount of radio signals received in such a heating conductor be available for a radio receiver or tuner, a leakage of the received signals to a direct current (DC) heater power supply or ground must be minimized. To this end, it was proposed to connect choke coils in the heating power supply lines so that the heating conductor will be floated or isolated from the heater power supply in high frequencies while allowing the heating power of DC current to pass through the choke coils. If the heating conductor antenna covers an FM broadcasting band only, choke coils having a relatively small inductance will suffice. However, such small coils will considerably decrease their impedance of radio signal leakage for an AM broadcasting band, thus reducing available antenna power in an AM band. On the other hand, if a large inductance of choke coils is employed, this will give a satisfactory antenna gain for an AM band but render a poor antenna reception sensitivity to an FM band because of an increased parasitic capacitance in such high inductance choke coils. In this view, the prior art has adopted an arrangement in which choke coils associated with a defogging heating conductor is made to have a relatively large inductance so that the heating conductor will serve only as an AM antenna. Separate antenna conductor elements are formed on the same window as that with the heating conductor to provide an FM antenna.
FIG. 1 shows an example of the prior art window glass antenna of this type. The antenna is arranged such that a large number of heater wires or strip conductors 2 constituting a defogging heater conductor 10 and serving as an AM antenna are formed in a defogging area of a rear glass window 1. The heater wires are subdivided into upper and lower wire groups which are powered from power supply buses 3 and 4 connected to first ends of the upper and lower wire groups, respectively. A connecting bus 5 is disposed along and commonly connects second or remote ends of the upper and lower wire groups to complete a heating current path. Since the heater wires 2 are used as an AM broadcast band reception antenna, a feeder cable 13 such as a coaxial cable is connected to a feeding point 12 provided in the connecting bus 5. The feeder cable 13 carries the received signal to an AM radio tuner mounted in the motor vehicle through a DC cut capacitor 14.
A separate FM antenna 6 is formed in a blank portion of the window 1 above the heater wires 2. It comprises a main antenna element 6a in the form of two parallel wires connected to each other in a substantial U shape, an auxiliary antenna element 6b disposed above and connected to the upper wire of the main antenna element 6a, another auxiliary antenna element 6c connected to the lower wire of the main antenna element 6a and facing the heater wires 2, and a folded element 6d bent back from the upper wire of the main antenna element and having a portion adjacent to the auxiliary antenna 6b. A feeding point 7 is formed on the folded portion 6d and connected to a feeder cable 8 for supplying an FM reception signal to an FM radio tuner through a DC cut capacitor 9.
A main DC power supply feeds a heating current to the power supply buses 3 and 4 through choke coils 16a and 16b magnetically coupled to each other. The choke coil 16a connected to a main power +B is negatively coupled to the choke coil 16b connected to a ground so that magnetic fluxes generated by the respective heating currents cancel each other within a core. Therefore, the core having a small volume can be operated in a nonsaturated state. A decoupling capacitor 20 is connected between a ground and a line connected to the main power +B to prevent power source noise from being superposed on the reception signal.
FIG. 2 illustrates another example of the prior art window glass antenna arrangement. In FIGS. 1 and 2, like references refer to like parts. In this example of FIG. 2, two FM antennas 36 and 38 are provided on spaces S1 and S2 in the window 1 above and below the heating conductor 10 as an AM antenna, respectively. The top FM antenna 36 includes, a coupling element 37 along the uppermost heating wire 2 for coupling with the heater conductor 10, and a feeding pad 7 connected to a feeder cable 35 for carrying AM and FM signals to an AM/FM tuner. The bottom FM antenna 38 also includes a coupling element 39 along the lowermost heating wire for coupling with signals from the heater conductor 10 and a feeding pad 40 connected to another feeder cable 41 for supplying the AM/FM tuner with AM and FM signals received in the heater conductor 10 and the bottom FM antenna conductor 38.
As noted from the foregoing, the prior art requires a separate antenna element or elements for receiving an FM band signals in addition to the heater conductor because the heater conductor only serves as an AM antenna. Such separate antenna element thus requires a relatively wide space for its mounting, as wide as about 120 to 150 mm above and/or below the heater conductor. Therefore, the prior art antenna arrangement is applicable only to a vehicle window glass having a large space; in small-sized cars such as two-box car, the rear window glass is limited in size, and further limited when it is mounted relatively up-right in the vehicle so that there is only a small top or bottom margin left in available for mounting an FM antenna conductor, once the heating conductor is formed on such small vehicle window.
Even in a motor vehicle having a relatively large window glass area and hence a large blank area, a TV broadcast reception antenna, an automobile telephone antenna, and the like are often formed in the blank portion. In this case, no further sufficient area is assured an FM antenna. Therefore, even if the glass window area is large, a glass antenna capable of receiving FM programs cannot often be formed.